Casein kinase 1 (CK1) is a family of evolutionarily conserved serine/threonine kinases including seven known members in vertebrates (CK1α, -β, -γ1, -γ2, -γ3, -δ and -ε). The CK1s contain a typical kinase domain followed by a C-terminal tail region, which has been implicated in the regulation of CK1 localization, substrate selectivity and kinase activity. Myriad proteins have been found to be phosphorylated by CK1s, which are involved in a wide range of cellular functions including vesicular trafficking, DNA damage repair, cell cycle progression, cytokinesis and circadian rhythms (reviewed by Gross and Anderson (1998); Vielhaber and Virshup (2001); Knippschild et al. (2005)). Moreover, CK1 family members (-α, -δ/ε and -γ) modulate the activities of major signaling pathways (for example, Wnt and Shh) through several mechanisms (Peters et al., 1999; Liu et al., 2002; Price and Kalderon, 2002; Davidson et al., 2005; Zeng et al., 2005 and reviewed by Price (2006)).
In mammals seven CK1 isoforms, namely CK1α, β, γ1-3, δ and ε, and several splice variants have been described. They all contain a highly conserved kinase domain, a short N-terminal domain of 6 to 76 amino acids and a highly variable C-terminal domain of 24 to more than 200 amino acids. The constitutive phosphotransferase activity of CK1 isoforms is tightly controlled by several mechanisms. For example, the closely related isoforms CK1δ and ε, which share a 98% identity at the amino acid level in their catalytic domain, are regulated by autophosphorylation, dephosphorylation and proteolytic cleavage. Members of the CK1 family are found in the nucleus, the cytoplasm and in the plasma membrane. By phosphorylating many different substrates bearing either a canonical or non-canonical consensus sequence, they modulate the activity of key regulator proteins involved in many cellular processes such as cell differentiation, cell proliferation, apoptosis, circadian rhythm, chromosome segregation, and vesicle transport.
The Pim kinase family contains three isoforms, Pim-1, Pim-2 and Pim-3, and has recently emerged as targets of interest in oncology and immune regulation. Ongoing studies have identified a role for these proteins in cell survival and proliferation, both functionally and mechanistically, and overexpression has been observed in a number of human cancers and inflammatory states.
Pim kinases suppress apoptosis and regulate cell-cycle progression. Elevated levels of Pim kinases have been reported in solid tumors such as prostate cancer and pancreatic cancer. Pim-1 was initially discovered in murine leukemia and several independent studies have shown this kinase to be upregulated in human prostate cancer. Pim-1, 2 and 3 make up a distinct and highly homologous family of serine/threonine kinases belonging to the calmodulin-dependent protein kinase-related (CAMK) family. In addition to the three gene-encoded proteins, translational variants have also been reported for Pim-1 and 2 resulting from utilization of alternative start codons. The name Pim refers to the original identification of the pim-1 gene as a frequent proviral insertion site in Moloney murine leukemia virus-induced T-cell lymphomas, and the gene encoding Pim-2 was subsequently found to have similar susceptibility. Pim-3, originally designated kinase induced by depolarization (KID)-1, was later renamed due to high sequence similarity to Pim-1 (71% identity at the amino acid level). Considering all three isoforms, Pim proteins are widely expressed with high levels in hematopoietic tissue and are aberrantly expressed in a variety of human malignancies. Pim kinases positively regulate cell survival and proliferation, affording therapeutic opportunities in oncology. The Pim protein kinases are frequently overexpressed in prostate cancer and certain forms of leukemia and lymphoma.
A role for Pim kinases in immune regulation has also been observed. Pim-2 has been reported to have enhanced levels of expression in a variety of inflammatory states and may function as a positive regulator of interleukin-6 (IL-6), whereby overexpression of the kinase augments stimulus-induced IL-6 levels. Pim-1 and 2 have also been implicated in cytokine-induced T-cell growth and survival. Comparing the sensitivity of stimulated T cells from Pim-1−/−Pim-2−/− mice to wild-type mice following treatment with the immunosuppressant rapamycin, it was found that T-cell activation was significantly impaired by Pim-1/Pim-2 deficiency, suggesting that Pim kinases promote lymphocyte growth and survival through a PI3K/AKT (PKB, protein kinase B)/mammalian target of rapamycin (mTOR)-independent pathway. Other parallel but independent functions and overlapping substrate specificity for proteins in these pathways have been reported as well, including the positive regulation of transcription of nuclear factor kappa-B (NF-κB)-responsive genes, which have implications in both inflammation and oncology. Therefore, Pim kinases are attractive targets for both therapeutic areas.
Further, Pim kinases have been reported to play a role in the protection of the ATP-binding cassette (ABC) transporter P-glycoprotein (Pgp; ABCB1) from proteolytic and proteasomal degradation. Pgp is known to mediate drug efflux, and, as such, inhibitors of Pim kinases may provide a novel approach to abrogating drug resistance.